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Task 4. Drainage System Description and Problem Descriptions <br /> According to the pre-app comments, there are three known drainage issues downstream of the site. The <br /> drainage issues downstream of the site are all far enough away that they should not be impacted by <br /> stormwater dispersed onsite. Based on the information available and all the resources available including <br /> visual inspection of the downstream flow path, there is no evidence of that any existing downstream <br /> drainage problems will be exacerbated by the proposed development. All flows are adequately dispersed <br /> onsite. <br /> SWMMWW Minimum Requirements <br /> This project adheres to the 2019 Stormwater Management Manual for Western Washington (SWMMWW) <br /> Minimum Requirements 1-5 and the City of Everett's Municipal Code. As such, the SWMMWW Minimum <br /> Requirements 1 through 5 are considered below for proposed site improvements. <br /> Minimum Requirement #1: Preparation of Stormwater Site Plans <br /> This memo along with the construction plans satisfy this minimum requirement. <br /> Minimum Requirement #2: Construction Stormwater Pollution Prevention (SWPP) <br /> A Stormwater Pollution Prevention Plan (SWPPP) has been prepared and submitted as a separate <br /> document. See Section 2 of this report for the SWPPP BMP Elements and the SWPPP for a complete <br /> discussion of erosion control BMPs. <br /> Minimum Requirement #3: Source Control of Pollution <br /> There will be no pollution elements anticipated that will require source control associated with the onsite <br /> project construction. If the need arises, proper BMPs will be implemented. <br /> Minimum Requirement #4: Preservation of Natural Drainage Systems and Outfalls <br /> The site has historically flowed very gently from west to east, with stormwater being adequately dispersed <br /> onsite. Natural drainage patterns will be maintained for the site with sheet flow dispersion adequately <br /> dispersing stormwater runoff onsite. <br /> Minimum Requirement #5: On-site Stormwater Management <br /> This project triggers Minimum Requirements #1-#5 and therefore shall use the LID BMPs from List #1 for <br /> all surfaces within each type of surface in List #1. The NRCS Soil Survey shows that the seasonal high <br /> groundwater elevation is roughly 18 to 36 inches below the existing ground level, and infiltration rates are <br /> likely very low (0.00-0.06 in/hr). Infiltration BMPs are infeasible for the site. Bioretention and rain garden <br /> BMPs are also infeasible because of the limited infiltration potential and high groundwater. Full dispersion <br /> is infeasible because of space constraints. Sheet Flow Dispersion will be utilized for all hard surfaces. New <br /> impervious roof and gravel areas are anticipated to disperse across 10'+ of native vegetation adjacent to <br /> the new impervious surfaces. BMP T5.12 Sheet Flow dispersion will be utilized to disperse stormwater <br /> runoff from the impervious surfaces to the maximum extent feasible given site conditions. Additionally, all <br /> disturbed areas that do not contain impervious surfaces are subject to BMP T5.13 — Amended Soils. For <br /> more detail, see the attached LID BMP List #1 evaluation in Appendix 3. <br /> If you have any additional questions during your review, please do not hesitate to contact us. <br /> Regards, tg P 4 <br /> wAsgf����� <br /> Tom Abbott, PE 56921 <br /> Project Manager ISTV' <br /> LDC, Inc. - HQ Office `sS10NAL <br /> Service ABOVE the Standard <br />